In conventional manner, electricity networks on the scale of a region, of a country, or of a continent, in which electric currents are transported over several tens, hundreds, or thousands of kilometers, are high voltage alternating current (AC) networks. Nowadays, trends in such networks are towards interconnecting infrastructures so as to obtain networks that are meshed, i.e. networks having a plurality of available paths between any two given points of the network. Furthermore, proposals have been made to develop networks or network portions using very high voltage direct current (DC), possibly integrated within meshed networks, together with portions of AC networks.
One of the problems in meshed networks lies in the possibility of transferring load currents between the different branches of the network in order to reorganize power flows, with this requiring electric circuits under high voltage to be opened or closed. This problem is even more acute with DC circuits. A conventional approach would be to use circuit breakers as breaker apparatuses, given that they are designed in particular to make it possible to open an electric circuit under load in which they are interposed. Nevertheless, circuit breakers are apparatuses that are complex, expensive, and voluminous, and they are intended for network protection functions, and would be under-used in such circumstances. In order to perform such load transfer functions, it may be helpful to use apparatuses of simpler design, such as disconnectors, even though those apparatuses are not primarily designed to break circuits that are under load. In the usual way in order to provide safety for equipment and personnel during interventions, disconnectors are to be found at each end of a line. It is thus appropriate to extract maximum benefit from those apparatuses.
In particular for high voltage circuits, it is also known to use so-called “metal-clad” apparatuses in which active breaker members are enclosed in a sealed enclosure filled with an insulating fluid. Such a fluid may be a gas, commonly sulfur hexafluoride (SF6), but it is also possible to use liquids or oils. The fluid is selected for its insulating character, in particular so as to present dielectric strength that is greater than that of dry air at equivalent pressure. Metal-clad apparatuses may be designed in particular so as to be more compact than apparatuses in which breaking and insulation are provided using air.
A conventional disconnector comprises in particular two electrodes that are held by insulating supports in stationary positions that are spaced apart from the peripheral wall of an enclosure, which wall is at ground potential. The electrodes are connected together electrically or separated electrically as a function of the position of a movable connection member forming part of one of the electrodes, e.g. a sliding tube actuated by a control. The tube is generally carried by one electrode, to which it is electrically connected, and separating the tube from the opposite electrode is likely to create an electric arc that may lengthen during the opening movement of the disconnector, while the tube is moving away from the opposite electrode. Conventionally, the disconnector has two pairs of electrical contacts carried by the tube and the two electrodes. The first pair is the pair that passes the nominal current in the fully closed position of the apparatus. This path for passing current, referred to as the “nominal path”, presents a path of least electrical resistance, thereby reducing conduction losses under steady conditions. This pair of contacts is associated with a second pair referred to as “arcing” contacts or as the secondary contact pair. The two contacts in this pair are caused to remain in close contact while the first pair is separated so as to avoid any arcing phenomenon on the first pair and thereby guarantee a good state of electrical conduction in the fully closed position. Conversely, the contacts of the secondary pair separate later on and an electric arc is struck between them. They need to be able to withstand such wear. Once the electric arc becomes long enough, and after a sufficient length of time, the electric arc becomes interrupted.
A disconnector is generally situated in an electricity substation. It is connected to the other elements of the substation, e.g. by busbars. On either side of a disconnector, other elements of the substation may be found such as a circuit breaker, a power transformer, an overhead bushing, . . . .
Such a disconnector without any specific device for facilitating breaking could be used to transfer those currents, and it would be capable of accommodating smaller stresses, but it would be inappropriate for circuits that present large loop impedances.
Under such circumstances, opening can lead to electric arcs that may stretch to considerable lengths, and that can lead to various problems. An arc that is too long between the connection member and the opposite electrode can degenerate and turn into a short circuit. For example, in a disconnector of the above-described type, an arc might strike between the live electrode and the wall of the enclosure connected to ground. In a less extreme situation, arc extinction times can become too long and can damage component parts and thus endanger the insulation of the system.
In certain circuit breakers designed to operate with AC at medium voltage, an arc splitter chamber is provided that is separate from the zone in which the movable connection member moves and that is offset away therefrom. An electric arc that forms, e.g. during opening of the circuit, is split into a multiplicity of arcs. Such circuit breakers require means to be provided for causing the arc to move away from the zone in which the movable member moves and towards the splitter chamber, e.g. by using a magnetic field, which may be created by permanent magnets or which may be induced by current flowing in a magnetic circuit. Either way, this aspect is complex to manage and requires numerous round trips during design stages in order to ensure that the arc goes into the splitter chamber, since the way the system behaves varies as a function of the magnitudes of the currents being switched. Furthermore, the splitter chamber constitutes additional bulk. For a metal-clad apparatus, this volume also needs to be insulated from the tank at ground potential in order to guarantee electrical insulation. This can lead to tanks of large size and costs that are disadvantageous.